1. Field of the Invention
The present invention relates to a hydraulic control apparatus which is preferably used to control, e.g., an automatic transmission of a vehicle, and in particular to the art of draining a frictional engagement element when an electric system fails.
2. Discussion of Related Art
There is known a hydraulic control apparatus which controls an operation of a hydraulically operated mechanical apparatus. One example of the hydraulic control apparatus is for hydraulically controlling a vehicle's automatic transmission including a plurality of hydraulically operated frictional coupling devices that are selectively operated to establish an appropriate one of a plurality of speed steps of the automatic transmission. The hydraulic control apparatus includes a plurality of electromagnetic valve devices which cooperate with each other to change the combination of respective operations of the frictional coupling devices and thereby establish one of the speed steps. Usually, the hydraulic control apparatus additionally includes an electronic control device which electrically controls the electromagnetic valve devices so as to establish one of multiple speed steps, e.g., five or six forward speed steps, depending upon an operating (e.g., running) state of the vehicle.
Meanwhile, it can be predicted that an electric system of a hydraulic control apparatus will halt its operation for some reason, i.e., so-called “fail”. In order to deal with the predictable fail, it is needed to introduce a fail-safe technique when the hydraulic control apparatus is originally designed. To this end, it has been proposed to introduce the art of preventing interlocking, into a hydraulic control apparatus for a vehicle's automatic transmission. This is disclosed by, e.g., Japanese Patent Publication No. P2001-248723A. The disclosed hydraulic control apparatus employs a fail-safe valve which shuts off an appropriate hydraulic passage, when at least two hydraulic pressures of (a) a signal hydraulic pressure that is outputted from a signal-hydraulic-pressure composing valve when two frictional engagement elements belonging to a first group are both operated or engaged, and (b) respective hydraulic pressures that are outputted to operate a plurality of frictional engagement elements belonging to a second group are supplied to the fail-safe valve. Therefore, the hydraulic control apparatus can stop, when the electric system fails, the supplying of hydraulic pressure to an appropriate frictional engagement element, and thereby prevent the occurrence of interlocking.